This invention relates to a new and improved method for determining residual oil saturation in a subsurface formation surrounding a borehole by carrying out borehole gravity measurements in accordance with a log-inject-log operation.
The log-inject-log operation has been utilized with thermal neutron capture logs of subsurface formations to determine residual oil saturation. U.S. Pat. Nos. 3,748,474; 3,757,575; 3,812,353 and 3,825,752 all disclose such use. Theoretically, such residual oil saturation may be represented by the following expressions: EQU .sub..SIGMA.formation =.sub..SIGMA.rock (1-.phi.)+.sub.93 fluid .phi.(1) EQU .sub..SIGMA.fluid =.sub..SIGMA.oil S.sub.oil +.sub..SIGMA.water S.sub.water ( 2) EQU S.sub.oil =1-S.sub.water ( 3)
where
.SIGMA. represents macroscopic absorption cross section,
S represents saturation (or volume fraction of a particular fluid in phase), and
.phi. represents porosity.
In many instances the macroscopic absorption cross section of the rock matrix (.SIGMA..sub.r) cannot be known or estimated with good accuracy. Strongly absorbing trace elements are usually too abundant in the rock matrix to permit an accurate determination of .SIGMA..sub.r from the rock's major constituents. The log-inject-log technique has been utilized to overcome this problem.